DRV401AMDWPREP datasheet(17/33 Pages) TI

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COMPENSATION DRIVER

MAX

OUT

65mA

EXTERNAL COMPENSATION COIL DRIVER

DRV401-EP

www.ti.com............................................................................................................................................................................................... SBVS104 – JANUARY 2008

The compensation coil driver provides the driving current for the compensation coil. A fully differential driver

stage offers high signal voltages to overcome the wire resistance of the coil with only +5 V supply. The

compensation coil is connected between ICOMP1 and ICOMP2, both generating an analog voltage across the coil

(see Figure 3) that turns into current from the wire resistance (and eventually from the inductance). The

compensation current represents the primary current transformed by the turns ratio. A shunt resistor is connected

in this loop and the high-precision difference amplifier translates the voltage from this shunt to an output voltage.

Both compensation driver outputs provide low impedance over a wide frequency range to ensure smooth

transitions between the closed-loop compensation frequency range and the high-frequency range, where the

primary winding directly couples the primary current into the compensation coil at a rate set by the winding ratio.

The two compensation driver outputs are designed with protection circuitry to handle inductive energy. However,

additional external protection diodes might be necessary for high current sensors.

For reliable operation, a wire break in the compensation circuit can be detected. If the feedback loop is broken,

the integrating filter drives the outputs ICOMP1 and ICOMP2 to the opposite rails. With one of these pins coming

within 300 mV to ground, a comparator tests for a minimum current flowing between ICOMP1 and ICOMP2. If this

current stays below the threshold current level for at least 100

µs, the ERROR pin is asserted active (low). The

threshold current level for this test is less than 57 mA at 25°C and 65 mA at –40°C, if the ICOMP pins are fully

railed (see the Typical Characteristics). For sensors with high winding resistance (compensation coil resistance +

RSHUNT) or connected to an external compensation driver, this function should be disabled by pulling the CCdiag

pin low.

Where:

VOUT equals the peak voltage between ICOMP1 and ICOMP2 at a 65mA drive current.

RMAX equals the sum of the coil and the shunt resistance.

An external driver for the compensation coil can be connected to the ICOMP1 and ICOMP2 outputs. To prevent a

wire break indication, CCdiag has to be asserted low.

An external driver can provide both a higher drive voltage and more drive current. It also moves the power

dissipation to the external transistors, thereby allowing a higher winding resistance in the compensation coil and

more current. Figure 4 shows a block diagram of an external compensation coil driver. To drive the buffer, either

one or both ICOMP outputs can be used. Note, however, that the additional voltage gain could cause instability of

the loop. Therefore, the internal gain can be reduced by approximately 8 dB by asserting the GAIN pin high.

RSHUNT is connected to GND to allow for a single-ended external compensation driver. The differential amplifier

can continue to sense the voltage, and used for the gain and over-range comparator or ERROR flag.

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DRV401AMDWPREP Datasheet(PDF) 17 Page - Texas Instruments

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